1. Field of the Invention
The invention concerns the output stages of a frequency synthesizer, stages which are designed to add large frequency steps to a frequency derived from preceding stages and which itself comprises smaller frequency steps.
2. Description of the Prior Art
The method is already known, from applicant's French patent No. 73 17650 to realize such a frequency summation by means of two current or voltage controlled oscillators, each controlled by a phase lock loop, a first loop dividing the frequency Fo+.DELTA. derived from preceding stages (.DELTA. being the sum of the low steps) by a variable integer N by means of a programmable divider and adding to it a standard frequency P which represents the high step, while a second loop multiplies the intermediate frequency Fi supplied by the first loop by the same factor N so as to give an output frequency Fs=Fo+.DELTA.+NP.
This solution is particularly useful when one desires to cover a range of output frequencies extending up to several gigahertz, the second oscillator then being of the YIG type. It has the advantage that, since this second oscillator is controlled on a sub-multiple of its own frequency, this frequency does not run the risk of being affected by the harmonics of Fi.
However, as N varies, the phase comparator of the second loop must obviously be of the sampler type, the frequency Fs of the output oscillator there being compared to a harmonic of variable rank N of the frequency Fi.
Now, it is known that a sampler, generally constituted by a capacitance-resistance and diodes bridge, equivalent to a switch through which the signal to be sampled is applied to a capacitor, the said switch being controlled by a pulse, generates in practice, owing to the necessary dissymmetry of pulses (supplied here by the harmonic generator), in addition to the wanted signal (oscillator direct-current control signal) whose level is very low, an additional direct-current componenet which results from the detection of pulses by the diodes. The level of this parasitic component evolves inside the frequency range and may, as regards certain frequency ranges, attain values greater than those of the wanted signal. Automatic control of the oscillator is not possible inside these ranges.
This drawback can be eliminated by comparing the beat between Fs and NFi obtained in the sampler to a carrier frequency comprising a fixed part Fo and, possibly, a variable part .DELTA. (the first loop thus generating a frequency Fi=NP+.DELTA. or, possibly, Fi=NP).
In such circumstances, the signal derived from the sampler is an alternating-current signal having a considerable amplitude and it is no longer affected by the parasitic signal.
On the other hand, the advantage previously mentioned is eliminated, since Fs is no longer a multiple of Fi.